In ultrasonic probes used as ultrasonic contact probes and ultrasonic flaw detection probes, acoustic impedance is an important factor for ensuring that the vibrations from a lead zirconate titanate (PZT) or other piezoelectric element are allowed to propagate efficiently or that vibrations are efficiently received from a subject. Therefore, materials are required in which the acoustic impedance thereof is roughly intermediate to that of the piezoelectric element and subject, and in recent years, configurations have been proposed that attempt to widen bandwidth and increase sensitivity of ultrasonic waves using multilayered acoustic matching layers.
In the case of laminating materials having different acoustic impedance, since an adhesive is typically used for this purpose, problems with respect to the acoustic impedance of the adhesive layer material, uniformity of coating thickness, delamination and working temperature are thought to occur.
In response to such problems, acoustic matching layers have been proposed that are obtained by laminating a plurality of layers having different acoustic impedance without using an adhesive. In this case, in addition to being able to prevent delamination and enhance mechanical strength, the issue is to reduce loss attributable to reflection of sound waves at boundary surfaces as much as possible.
In response to this issue, Patent Document 1 describes that, by composing a first layer 13a in the form of a porous body (paragraph 0052) and coating with a material of a second layer 13b after sintering the first layer as shown in FIG. 3 of Patent Document 1, a portion of the second layer enters micropores of the first layer, and by further sintering in this state, peel strength between the first layer 13a and the second layer 13b is enhanced by an anchoring effect (paragraphs 0060-0061).
Although the method according to this Patent Document 1 is able to enhance peel strength, a well-defined boundary is still present between each layer.
Patent Document 2 describes a carbon-based acoustic plate that can be used as an acoustic matching plate. Example 2 of Patent Document 2 describes that, by mixing a certain ratio of graphite particles into a furan resin serving as a carbon source of amorphous carbon as a skeleton, further incrementally mixing in polymethyl methacrylate particles, which are lost during the course of carbonization resulting in the formation of pores, into each layer in different ratios, and subjecting to heat treatment in a nitrogen atmosphere to carbonize the furan resin and eliminate the polymethyl methacrylate particles to form pores, a carbon-based porous body can be obtained in which acoustic impedance changes in the direction of thickness and boundaries between two adjacent layers are not present.
However, according to the technique described in Patent Document 2, since the same filler particles in the form of graphite particles are incorporated into all layers at the same ratio, and changes in acoustic impedance are realized due to differences in porosity, there are limitations on the amount of change in acoustic impedance in the direction of thickness. In addition, Patent Document 2 does not consider the issues of bending and/or delamination during the course of carbonization and pore formation.